The mainstream of refrigerators produced in recent years is constituted by large models with a capacity of 350 liters or over, and the majority of those refrigerators are represented by inverter-controlled refrigerators with highly efficient variable compressor speed. Many of compressors for those refrigerators adopt brushless DC motor having a rotor with permanent magnet, for the purpose of achieving higher efficiency. Moreover, since the brushless DC motor is installed under environments of high temperature, high pressure, refrigerant ambiance, and oil ambiance in the compressor, there is no way to use any Hall element. For that reason, a method of detecting the rotor position from the voltage induced on the stator is often used.
FIG. 21 is a block diagram of a conventional brushless DC motor drive unit, disclosed on Japanese Laid-Open Patent Application No. H9-88837. In the diagram, the commercial power source 101 is an AC power supply unit with a frequency of 50 Hz or 60 Hz and a voltage of 100V in Japan. The rectifier circuit 102 is composed of bridge-connected rectifying diodes 102a to 102d and smoothing electrolytic capacitors 102e, 102f. The circuit in the diagram, which is a voltage doubler rectifier circuit, provides a DC voltage of 280V from AC 100V input. The inverter circuit 103 is constructed with 6 switching elements 103a, 103b, 103c, 103d, 103e, 103f formed in three-phase bridge. The respective switching elements have parallel diodes in reverse direction for reflux current, but they are omitted in this diagram. The brushless DC motor 104 is composed of a rotor 104a having a permanent magnet and a stator 104b having a three-phase winding. The rotor 104a can be rotated as a three-phase AC current produced by the inverter 103 flows through the three-phase winding of the stator 104b. The rotating motion of the rotor 104a is changed into alternating motion by a crankshaft (not illustrated), to drive the compressor for compressing refrigerant.
The back electromotive voltage detecting circuit 105 detects rotor position, from the voltage induced on the stator 104b with rotations of the rotor 104a. The commuting circuit 106 makes logical signal conversion to the output signal of the back electromotive voltage detecting circuit 105, to produce a signal for driving the switching elements of the inverter 103.
The synchronous driving circuit 107 produces at prescribed frequency a signal of the same shape as the signal generated in the commuting circuit 106, to synchronously driving the brushless DC motor 104. The load state judging circuit 108 judges the load state in which the brushless DC motor 104 is being operated. The switching circuit 109 selects, with the output of the load state judging circuit 108, either the commuting circuit 106 or the synchronous driving circuit 107, to drive the brushless DC motor 104. The drive circuit 110 drives the switching elements of the inverter 103, with a signal from the switching circuit 109.
In the case where the load detected by the load state judging circuit 108 is an ordinary load, the driving is made with the commuting circuit 106. In that case, the back electromotive voltage detecting circuit 105 detects the rotor position, and the commuting circuit 106 produces commuting pattern for driving the inverter 103 based on the rotor position. This commuting pattern is supplied through the switching circuit 109 to the drive circuit 110, to drive the switching elements of the inverter 103. As a result, the brushless DC motor 104 is driven in correspondence to the rotor position. Namely, the brushless DC motor 104 is driven as an ordinary brushless DC motor.
As the load increases, the brushless DC motor 104 reduces in rotating speed because of its characteristics. This state is judged by the load state judging circuit 108 as a high-load state, and the output of the switching circuit 109 is switched to the signal from the synchronous driving circuit 107. Namely, the brushless DC motor 104 is driven as a synchronous motor, to prevent reduction of rotating speed at high load.